BT has announced it will install ADSL2+ broadband equipment in 150 additional telephone exchanges, bringing up-to-24Mbps downstream speeds to an extra three million citizens.

The rollout will take total coverage of ADSL2+ up to two-thirds of the British population, and will be spread thinly across the UK.

However, some areas will benefit from the expansion more than others — in particular the home counties, the south coast, the West Midlands, the Liverpool-Manchester belt and Northern Ireland. Most of Wales, East Anglia, the south-west, the north-west and Scotland will miss out.

London, Manchester and Leeds already have close to ubiquitous ADSL2+.

BT would not say when it expected to complete the installation. But it did say it would meet its earlier roll-out target for ADSL2+ of 75 percent of the population by the first part of next year.

The announcements were made on Monday at a private event in London called the BT ISP Forum. "We are delighted to confirm this next tranche of exchanges, but it doesn't complete the year's roll-out schedule. We expect to announce more exchanges this summer as we remain fully committed to our intention to deliver WBC [Wholesale Broadband Connect] to up to 75 percent of UK homes and businesses by Spring 2011," said BT Wholesale's general manager for broadband products Emma Elshof at the event.

Wholesale Broadband Connect is a service to ISPs from BT Wholesale that is a prerequisite for ADSL2+ rollout.

Though ADSL2+ speeds decrease markedly with distance from the exchange, the installation of the new equipment offers the prospect of slightly faster broadband speeds for most citizens. The company's maximum download speed with ADSL2 was 8Mbps.

BT has been steadily rolling out ADSL2+ across its footprint in tandem with its 21st Century Network (21CN), a programme that has seen BT start to radically change and simplify its core network. Though the 21CN project has been downsized, it is supporting faster access speeds.

21CN has enabled BT to put concrete numbers on the speeds it can offer. BT said Monday that from 7 June, BT Wholesale will promise to ISPs taking its copper-based services that 4Mbps downstream throughput on consumer lines and 8Mbps downstream throughput on business lines will be achieved for a set percentage of the day. Guarantees will also be available for fibre broadband lines. ISPs will be able to log a fault if performance falls below those levels, BT said.

As well as supporting ADSL2+, the fact that the exchanges in question have been connected to 21CN will also mean they can be upgraded to super-fast broadband — either fibre-to-the-home (FTTH) or its slower counterpart, fibre-to-the-cabinet (FTTC). These will give 100Mbps and 40Mbps downstream speeds, respectively.

As well as offering the faster download speeds, BT Wholesale is testing faster upload speeds. This is being achieved through the use of Annex M of the ADSL2+ standard, which can enable 2.5Mbps upstream for homes close to their exchange. Ten of BT's wholesale ISP customers are testing Annex M in small-scale trials.